It has washed away over the past couple years since the plants that were there previously to keep soil in place were all destroyed. The side of the yard has a relatively steep slope in that spot and there's a rain gutter that empties right at the top of the slope - sometimes with a lot of force. The dirt isn't anywhere nearby to be seen; at least not in a form that could be easily shoveled back into place. I wouldn't be too surprised to learn that the dog ate it just to spite me. :)

If you have a big indentation to fill, fill dirt is fine for the majority of it but I would opt for a better quality soil for the last 8 inches or so to accommodate any plantings.

lazy_gardens thinks you're in Arizona where everything is flat and we don't know what the word 'rain' means. :-))

If you are just filling in a hole to reinforce the foundation, any old dirt should do. Normally a tree's roots will extend deeply down, not out.

Some dog breeds are serious diggers, others chewers, others barkers.....life is all about trade-offs. I love my Cavalier King Chas spaniel but this boy has some serious anxiety/hyperactivity issues. [groan].... but he does not dig or chew. Good luck with the tree planting and dog days.

Thank you! We definitely know what a lot of rain is in MN. We planted a rain garden in the back yard to catch all the runoff that swiftly runs downhill. That's probably where all the dirt ended up as a final destination. :)

I'll most likely end up getting about half clean fill just to raise it back up since it's a lot cheaper, then purchase good quality soil and compost for the top half (12"+) for the benefit of plants and the future tree.

Dogs won't dig stuff that will hurt their claws, so a spaced layer of coarse fill (such as broken concrete or river rock or broken pavers) a bit under the top growing layer may take the fun out of digging or at lease displace it to other spots. This is only what you'd do to prevent digging under a gate, just taking some plants in mind.

"Normally a tree's roots will extend deeply down, not out."

That is completely wrong. Most tree roots are going to be within the top 24 inches of soil and extend past the drip line of the canopy by 2-3 canopy radius lengths.

I think it's more regional than that and I can, of course, only speak of my experience in the southwestern desert. Our trees must put down deep roots or they will blow over in a monsoon storm. So, with respect Nil13, I don't believe it is *completely* wrong.

"Unless the dog has the ability to use buckets or wheelbarrows to haul the dirt off, you should find enough dirt around the hole to fill it back in."

lol

My dogs like to track all that dirt inside the house.

Guess that depends where the OP is located and the tree - but yea tree roots typically extend outward of the dripline. Any deeper roots would be searching for water and nutrients not found near the surface roots.

Very few trees produce tap roots or a deeply penetrating vertical root system but it does tend to be more common in very arid areas where the trees are seeking underground moisture sources. So in AZ, that statement is more or less correct :-) But I'd omit the "normally" -- it is the exception, not the rule. The vast majority of trees - and even those in well irrigated gardens in AZ - will have spreading, very horizontal root development, with the bulk of the root system withing the top 12-16" of the soil and most fine feeder roots just below the soil surface.

I appreciate the corrections. :-)